Method of connecting a steel blank to a tungsten bit body



Oct. 14, 1969 R. FEENSTRA 3,471,921

METHOD OF CONNECTING A STEEL BLANK TO A TUNGSTEN BIT BODY Filed NOV. 16,1966 I a '-'-f 3\ FIG. I

INVENTOR:

ROBIJN FEENSTRA HIS AGENT United States Patent US. Cl. 29473.ll 6 ClaimsABSTRACT OF THE DISCLOSURE A steel blank is connected to a tungsten bitbody by bonding the contact planes of the body and the blank together ata temperature that is less than the critical temperature of the steelblank so as to avoid the tremendous volume changes that occur in steelat this temperature.

The present invention relates to drill bits employed in drilling oil andgas wells and pertains more particularly to a method of connecting asteel blank to a bit body consisting of a mass of sintered tungsten(e.g., tungsten powder and/or tungsten carbide powder). In particular,the present invention relates to a manner of providing the sintered bodyof a diamond bit for use in deep well drilling, with a steel tool joint,sub or shank which is suitable for connecting the sintered bit body to adrill string.

Diamond bits are manufactured by filling a graphite mold of the bitbody, with tungsten powder and/or the desired places within the mold. Toconnect the mass to the drill string, a steel blank, which is laterprovided with a screw thread so as to act as a sub or shank, is placedin the mass of powder filling the mold, after which the mold with thepowder and the blank is placed in a suit able furnace. A bindermaterial, such as german silver, is placed on the top of the powder, andthe furnace is heated to sintering temperature which is about 1l30 C.,and, of course, well below the temperature at which the diamonds wouldbe adversely aifected.

At sintering temperature, the binder material liquefies and flows intothe pore space of the powder material, as well as between the powdermaterial and the wall of the steel blank. Cooling down of the mold withits contents causes solidification of the binder material, therebyforming an integral mass of the powder material containing the diamonds,to which mass the steel blank is attached. A screw thread is thereaftercut on the blank, thereby converting it into a sub or tool joint pinwhich is suitable to be connected to the lower end of a drill collar.

It has been found, however, that due to the difference in the expansioncoefficients of the sintered mass and the steel blank the sintered masswill be liable to crack during the cooling period. In addition, thisweakens the bond between the blank and the sintered mass, and it alsodistorts the sintered mass which will adversely affect the cuttingproperties of the diamonds carried by the sintered mass.

A possible solution for the above problem is the use of a number ofsmall metal plugs, provided with internal screw threads, which plugs aresintered together with the powder, and allow the application of a flangeconnected to a sub, which flange is provided with openings carryingbolts for connecting this flange to the sintered bit body. Since thedimensions of these plugs are small compared to the dimensions of thesintered mass, cracking of the sintered mass does not occur duringcooling. Although this construction is very useful for testing diamondbits in the laboratory, it is unsuitable for field use on account of theuse of the bolt connections.

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It has also been proposed to sinter the powder in the mold together withthe diamonds, but without the steel blank. After the sintering processis over and the sintered mass has cooled down, the steel blank is brazedwith copper to the sintered mass by heating the whole assembly to atemperature which is lower than the sintering temperature. This willresult, depending on the construction of the blank and the sinteredmass, in cracking of the sintered mass or breaking of the bond betweenthe blank and the sintered mass. If the blank is placed in a groovearranged in the sintered mass, even both phenomena may occur.

Accordingly, the invention is concerned with connecting a steel blankmember to a tungsten bit body member consisting of a mass of sinteredtungsten powder and/or tungsten carbide powder, by such a method as toobtain a strong bond between the steel blank and the sintered mass,without distortion or cracking of the sintered mass or breaking of thebond between the steel blank and the sintered mass.

According to the invention, the method of connecting a steel blank to atungsten bit body consists in bonding the contact planes of the mass andthe blank together at a temperature which is less than 723 C., thelower. critical temperature of the steel blank. Critical temperatures,also called critical points, of steel are those temperatures at whichstructural changes take place in the steel while it is in the solidcondition. These critical temperatures vary with the rate of heating orcooling of the steel and with different alloys. The lower criticaltemperature for a steel is that point at which pearlite begins to changeinto austenite. This lower critical temperature occurs about 723 C. forsome pure iron-carbon alloys. The structural change of the steel frompearlite to austenite and vice-versa is accompanied by a volume changeof very specific nature. This volume change may be as much as 1.6% overcertain temperature ranges.

The bond may be obtained by the application of a bonding compositionsuch as a resin composition which cures at a temperature below 723 C.,or by a brazing composition, which has a melting range below 723 C. Inother words, the body may be obtained by the application of a bondingcomposition or agent having an eifective bonding temperature that isless than the lower critical temperature of the steel member. Effectivebonding temperature as used herein is that temperature to which thebonding composition must be raised during the course of establishing anelfective bond. The bonding composition may be either a resincomposition that cures at a temperature below the lower criticaltemperature of the steel member or a brazing or soldering compositionhaving a melting point below said critical temperature.

The invention may be carried into practice in various ways, but onespecific embodiment will now be described by way of example withreference to the accompanying drawing which shows a longitudinal sectionof a diamond drill bit which is suitable to drill deep wells throughhard rocks.

FIGURE 1 is a section through the assembled tool bit; and

FIGURE 2 is a section through the bit body as molded.

The bit body includes a sintered mass 1 carrying the diamonds 2 and hasbeen formed by means of a mold 3. The mold consists of graphite, and maybe formed by turning a solid block of graphite on a lathe and cutting anegative form of the desired bit design in the block.

The required number of diamonds 2 are then distributed according to adesired pattern in the mold, whereafter the mold is filled with powdermaterial, such as tungsten powder or tungsten carbide, suitable to bebonded together by a binder material. If desired, the outer wall 4 ofthe mass 1 may be formed by tungsten carbide powder, whereas theinterior layer is constituted by tungsten powder. An annular channel 6may be formed in the tungsten powder mass 5 by placing a graphite ring6a of the dimensions corresponding to the channel 6 in the mass oftungsten powder 5.

After the mold 3 has been filled with the required amounts of tungsten 5and tungsten carbide 4, an amount of suitable binder material, such asgerman silver consisting of 65 vol. percent copper, 18 vol. percentnickel and 17 vol. percent zinc, is placed on top of the tungstencarbide powder 4 and/or tungsten powder 5. Thereafter the mold 3 withits contents is placed in a suitable sintering furnace in which the moldis heated in a neutral atmosphere to a temperature higher than themelting temperature of the binder material. When applying german silveras a binder, the sintering temperature of 1130 C. will be suflicientsince the melting range of this binder is 1120 C.

On melting, the binder material sweeps through the pore space of thepowdered masses 4 and 5 and is evenly distributed thereover. During thecooling period following the heating period in the furnace, the bindersolidifies, thereby strongly bonding the powder particles of the mass 1to an integral unit. Since the diamonds 2 are for the greater partenclosed by the sintered particles, these diamonds are firmly retainedin the sintered mass 1.

After the sintered mass 1 has cooled down to ambient temperature, thegraphite mold 3, as well as the ring in the channel 6, if present, isremoved from the body 1 (e.g., by destroying). Thereafter the body 1 isplaced on a lathe and the form and dimensions of the channel 6 arebrought into accordance with the lower end of the metal blank 7 by meansof a cutting tool.

The metal blank 7 comprises a lower part 8 which is preferably of across-section which is substantially of the same shape as thecross-section of the body 1. The clearance between the blank 7 and thebody 1 has been calculated such that at brazing temperature, it has aclearance suitable for brazing (e.g., about 0.1 millimeter).

The upper part 9 of the blank 7 has an external screw thread 10, whichenables the body 1 to be connected by screwing action to a drill collaror drill string suitable so that it can be lowered into a borehole andbe rotated therein under a load sufiiciently high to have the diamonds 2exert a scraping action on the bottom of this hole for increasing thedepth thereof. The screw thread 10 is not cut into the blank 7 untilafter the latter has been connected to the body 1.

After the blank 7 has been placed in the channel 6, and the body 1 andthe blank 7 have been put into a heating furnace and have been heated toa temperature lower than 723 C., the critical temperature of the steelblank, a very small clearance will remain between the outer wall of theblank 7 and the inner wall of the channel 6. An amount of lowtemperature brazing composition is placed at one side of the blank 7just above the clearance between the blank 7 and the channel 6. Thebrazing composition, on melting, flows into the clearance and fills itcompletely.

A suitable brazing composition is formed by a silver solder comprising50 vol. percent silver, 18 vol. percent cadmium, 16.5 vol. percent zincand 15.5 vol. percent copper. Since the flow point of this solder isabout 635 C., a furnace temperature between 635 C. and 723 C. issufiicient to heat the solder to a temperature above its melting range.The space around the contact planes which are to be bonded by the solderis filled with nitrogen gas to decrease the danger of carbonisation ofthe steel.

By gradually cooling down the furnace to ambient temabove 723 C., itslower critical temperature, this temperature being the bottomtemperature of a temperature range over which pearlite is converted intoaustenite when the temperature of the steel is raised, and over whichaustenite is converted into pearlite when the temperature of the steelis lowered.

These conversions from pearlite to austenite and viceversa areaccompanied by volume changes of a very specific nature. Thus, oncooling down steel over the temperature range of maximum 910 C. to 723C., an increase of volume of 1.6% will occur. Since the shrinking actionof the sintered mass, however, is quite normal when cooling it down froma high temperature, it will become apparent that in a construction suchas the present diamond bit, in which the steel blank and the sinteredbody have a very'close fit, undesired stresses will occur when coolingthis construction over the above-mentioned temperature range of 910 C.to 723 C., in particular when a bond has already been formed between thesteel blank and the sintered-body.

It will be clear that, by applying methods in which the bond between thecontact planes of the steel blank and the sintered body is formed at atemperature lower than 723 C., the lower critical temperature of thesteel member, no problems regarding overstressing of the constructionwill occur. One of the methods which makes use of a low-temperaturebrazing solder has already been described above. It will be appreciatedthat the invention is not restricted to the particular type of solder orbonding composition used in the above explanation of the invention, butthat any other type of brazing solder or bonding composition may be usedwhich has a flow point or melting range which is lower than 723 C., thelower critical temperature of the steel member, and is capable offorming a sufiiciently strong bond between the available areas of thecontact planes of the blank and the body.

Another way of obtaining a bond between the blank and the body withoutcreating undesired stresses within these components of the bit in theuse of a bonding composition such as a resin which will cure at atemperature lower than 723 C., the lower critical temperature of thesteel member, and sufiiciently adhere to the blank as well as to thebody to withstand the forces which are exerted in the bond between thecontact planes of the bit components when the bit is in operation indrilling a hole in an underground formation. One example of such resinsis a glycidyl polyether of 2,2-bis(4-hydroxyphenyl)propane, known underthe tradename of Epon resins, which can be cured, depending on thecuring agent and the curing period, at any temperature between roomtemperature and 200 C.

It will be obvious that various modifications may be made in the methodand apparatus according to the invention and that the specific detailsof the method and apparatus as described herein are merely illustrative.

I claim as my invention:

1. Method of minimizing dilferential expansion and contraction between asteel blank member and a sintered tungsten earth drill bit member whilepermanently bonding said members together, comprising the steps of:

forming one of said members with a recessed portion;

forming the other of said members with a portion dimensioned to matewith said recessed portion in spaced relationship at maximum bondingtemperature;

placing said portion of said other member within said recessed portion;maintaining said portions in spaced relation at said maximum bondingtemperature;

providing a bonding composition having an efiective bonding temperaturethat is less than the minimum critical temperature of said steel member;

placing said bonding composition in the space formed upon the mating ofsaid members;

applying heat to said bonding composition at a temperature 'sufficientto bring said bonding composition substantially to its effective bondingtemperature; and cooling said members to ambient temperature to effect apermanent bond between said members, thereby minimizing the stressing ofsaid members, and eliminating the cracking and distortion of said bitmember and the weakening of said bond. 2. The method of claim 1including the step of: adding diamonds to the surface of said sinteredtungsten bit, said diamond inclusions having at least a portion thereofexposed. 3. The method of claim 1 including the further step of:exposing at least the mating portions of said members with an inert gasduring at least said heating step. 4. The method of claim 1 wherein thestep of providing said bonding composition comprises employing a resinwhich will cure at a temperature of less than the critical temperatureof said steel member.

5. The method of claim 3 wherein the step of providing said bondingcomposition comprises employing a.

brazing solder having a melting point which is lower than the criticaltemperature of said steel member.

6. The method of claim 1 wherein the step of applying heat to saidbonding composition comprises applying heat to at least one of saidmembers.

References Cited JOHN F. CAMPBELL, Primary Examiner J. L. CLINE,Assistant Examiner US. Cl. X.R.

